A wave of cubes sent by the Shakri, after being dormant for a year, induced cardiac arrest in a third of Earth's population, as well as the Eleventh Doctor. The Doctor was able to survive with only one heart working, although according to him, the Doctor wouldn't last much longer. Amy Pond restarted his non-functioning heart with a defibrillator. The Doctor was then able to make the cubes reverse the process and save the humans. (TV: The Power of Three)